The invention relates to reducing and releasing trapped pressures inside downhole tools.
In completing a well, various types of downhole tools are run into the wellbore. One type of tool is a perforating gun string, which typically includes one or more gun sections. If multiple gun sections are present, spacer sections can be located between the gun sections to provide a desired spacing between the gun sections. Various different types of perforating guns can be used. One type is the hollow carrier gun, which includes a hollow carrier containing shaped charges. The hollow carrier provides a sealed chamber that protects the shaped charges from wellbore fluids and high pressures inside a wellbore. Another type of perforating gun does not employ hollow carriers, but instead uses capsule shaped charges that are contained in individual sealed capsules and which are typically carried on strips or other types of carriers.
In operation, the perforating gun string is lowered into the wellbore to a desired depth. In a hollow carrier gun, the inside chamber of the hollow carrier in which the shaped charges are contained is typically at atmospheric pressure. The outside of the hollow carrier is exposed to the pressure of wellbore fluids, which can be at hydrostatic pressure or a higher formation pressure. When the gun string is detonated, explosive gases at a high pressure are created inside the gun, with a portion of the gases entering the wellbore through perforated openings of the hollow gun carrier. After the explosive gases cool, they are overcome by the typically higher wellbore pressures, with the wellbore fluids entering the guns through the perforated openings in the hollow gun carrier. After filling the hollow gun carriers, the wellbore fluids fill spacer subs until pressures are equalized at the wellbore pressure.
As the wellbore fluid enters the spacer subs, debris and explosive gases from the perforating operation may also be carried into the spacer subs. As a result, the spacer subs are filled with compressed air, compressed explosive gases, wellbore fluids, and debris (e.g., gun debris, formation debris or other debris). Typically, the heavier components of the mixture start to settle out to the bottom of each spacer sub.
As the gun string is retrieved from the wellbore, the hydrostatic pressure outside the gun string gradually decreases. The compressed air and explosive gases that are inside the spacer subs expand to expel liquids, gases and debris from the upper and lower ends of the spacer subs. However, some of the larger pieces of debris may create plugs that prevent further expulsion of fluids and debris, which then limits the expansion of the remainder of the liquid and explosive gases as the pressure outside the gun string continues to decrease as the gun string is raised. Consequently, relatively high pressure (along with gases and well liquids) may be trapped in some spacer subs.
At the well surface, the pressure outside a retrieved gun string is atmospheric pressure. However, the trapped pressure inside the spacer subs can be much greater than the atmospheric pressure. This poses a safety concern as the gun string is disassembled at the well surface. The trapped pressure inside the guns may be suddenly and unexpectedly released, causing injury to well personnel. Also, the trapped well liquids and gases can also pose contamination concerns for the environment.
The chance of encountering trapped pressure inside a retrieved gun string increases as the number and length of spacer subs increase, as the hydrostatic pressure encountered increases, and as the wellbore fluids become thicker, heavier and more viscous.
A need thus exists for an improved method and apparatus to reduce trapped pressure inside downhole tools, such as perforating guns.
In general, according to one embodiment, a perforating gun string comprises a gun section and a filter sub connected to the gun section, the filter sub containing a filter having a multilayered assembly containing plural screens.
In general, according to another embodiment, an apparatus for use in a gun string having a gun section in a wellbore comprises a body adapted to be connected to the gun section and a multilayered assembly having plural screens inside the body to filter out debris.
Other or alternative features will become apparent from the following description, from the drawings, and from the claims.